This project is devoted to studies on age related changes in immunologic and biochemical parameters of lymphoid cell function. The areas studied to date have been: a) the role of accessory cells and their secretory products in age related decline in the response to T cell mitogens; b) the age related change in thymidine kinase activity in lymphoid cells; and c) the response of lymphocytes from young and aged animals to aromatic hydrocarbons by induction of aromatic hydrocarbon hydroxylase.